1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and a method of heating a fixing member, and more particularly, to a fixing device for fixing a toner image on a recording medium, an image forming apparatus including the fixing device, and a method of heating a fixing member that fixes a toner image on a recording medium.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a flexible, endless fixing belt formed into a loop and a resistant heat generator provided inside the loop formed by the fixing belt to heat the fixing belt, to shorten a warm-up time or a time to first print (hereinafter also “first print time”). Specifically, the resistant heat generator faces the inner circumferential surface of the fixing belt across a slight gap through which radiation heat generated by the resistant heat generator is transmitted to the fixing belt quickly. A pressing roller presses against a nip formation member also provided inside the loop formed by the fixing belt via the fixing belt to form a nip between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. As the recording medium bearing the toner image passes through the nip, the fixing belt, heated by radiation heat generated by the resistant heat generator, and the pressing roller together apply heat and pressure to the recording medium to fix the toner image on the recording medium.
With the above configuration, the slight gap provided between the resistant heat generator and the fixing belt prevents wear of the resistant heat generator and the fixing belt while at the same time providing the shortened warm-up time and the shortened first print time described above. Accordingly, even when the fixing belt rotates at a high speed, the resistant heat generator heats the fixing belt to a desired fixing temperature with reduced wear of the fixing belt and the resistant heat generator.
However, the fixing device including the resistant heat generator and the fixing belt has a drawback in that rotation and vibration of the pressing roller repeatedly applies mechanical stress to the resistant heat generator via the fixing belt, which bends the resistant heat generator. The repeated bending of the resistant heat generator causes fatigue failure and concomitant breakage or disconnection of the wiring of the resistant heat generator, resulting in faulty heating of the fixing belt.
To address this problem, the fixing device may further include a support member that supports the resistant heat generator and the nip formation member so that the resistant heat generator resists the mechanical stress applied by the pressing roller. However, since a lubricant (e.g., grease or oil) is applied between the fixing belt and the nip formation member to reduce friction therebetween, the lubricant may have the effect of increasing rotational torque of a driver that drives and rotates the pressing roller. Specifically, when the fixing device is not yet warmed up, the lubricant is also not warmed and therefore has an increased viscosity that increases frictional resistance between the nip formation member and the fixing belt sliding over the nip formation member.
To address this problem, pressure applied by the pressing roller to the fixing belt may be changed based on the temperature of the fixing belt detected by a thermistor. For example, the pressing roller applies reduced pressure to the fixing belt at a low temperature of the fixing belt, which increases the viscosity of the lubricant, and applies increased pressure to the fixing belt at a high temperature of the fixing belt, which reduces the viscosity of the lubricant. However, this configuration may require a complicated pressure adjustment mechanism that changes the pressure applied by the pressing roller in multiple steps based on the temperature detected by the thermistor as well as a complicated control mechanism to control the pressure adjustment mechanism.
Alternatively, a spring may be used to press the resistant heat generator against the fixing belt with increased pressure that reduces thermal resistance between the fixing belt and the resistant heat generator contacting each other. However, this configuration may also require a complicated pressure adjustment mechanism that changes bias applied by the spring in accordance with increased frictional resistance between the resistant heat generator and the fixing belt sliding over the resistant heat generator caused by the increased viscosity of the lubricant at a low temperature.